Hierarchically nano-assembled oxygen carriers for chemical looping

Abstract

Chemical looping (CL) is an increasingly important approach for net-zero energy conversion, power generation, and fine chemical production. By leveraging lattice oxygen in solid oxygen carriers (OCs) instead of gas-phase oxygen, a CL process intensifies redox transformations and avoids complex product separation. However, industrial CL processes demand structurally stable and chemically controllable OCs with high oxygen capacity, regulatable reactivity, and structural integrity even under harsh operational conditions. Currently, several key challenges exist: 1) Cation diffusion and agglomeration of OCs during cyclic lattice oxygen release and restoration; 2) sintering and inactivation of OCs under harsh reaction conditions (e.g., high temperatures); and 3) lack of effective OCs towards CL-coupled tandem catalysis. A potential strategy to resolve these challenges is to assemble hierarchical OCs that integrate oxygen storage materials, catalysts and inert supports from the nanoscale. In this regard, this review aims to critically assess the advanced design and synthesis of such hierarchical structures with embedded and core–shell configurations, and then clarify the relationships between the structural hierarchy and superior CL performance over a wide range of applications (from syngas to biomass conversion). The unique roles of confinement effects and strong metal–support interactions in hierarchical OCs are analyzed to explain the enhanced durability and CL activity. Based on current synthesis approaches, this review further identifies the challenges and future perspectives of hierarchical OCs for CL tandem catalysis, e.g. Fischer–Tropsch synthesis, alkane dehydrogenation, and CO₂ hydrogenation, towards CL-based “net-zero” applications.